1. Field of the Invention
The present invention relates generally to determining the drilling path of a borehole, and particularly to tracking and guiding the drilling of a borehole between a specified borehole entry and exit locations.
2. Technical Background
Various well-known drilling techniques have been used to place underground transmission lines, communication lines, pipelines, etc., over, around, between or under, obstacles of various types. To traverse an obstacle, a borehole must be drilled under the obstacle from a specified entry point to a specified exit location. Subsequently, the borehole may receive, e.g., a casing that can be used as a pipeline or a “raceway” for various kinds of cables. (The cables may be configured as power transmission lines, communication lines, or the like). What is needed is a system and method that allows a borehole to be drilled along a precise path so that boreholes can be accurately placed in locations that are encumbered with one or more physical obstacles (such as buildings, rivers, streets, rail lines, airport runways, and previously placed sewer lines and underground cabling, etc.). In other words, the aforementioned obstacles make the digging of a trench impossible or prohibitively expensive.
To be more specific, when a borehole is being drilled in such locations, the drilling apparatus (creating the borehole) must be carefully controlled so that the borehole closely follows a predetermined path comprising the entry point, borehole path and the exit point (i.e., the “prescribed drilling proposal”). While the task of establishing the entry point is easy enough, the borehole must remain within a predetermined right of way as it passes under the aforementioned obstacles. Moreover, the borehole exit point (like the borehole entry point) is typically located within a precisely defined area on the opposite side of the obstacle.
In one approach that has been considered, a plurality of orthogonal gyroscope (“gyro”) sensors were employed to measure the three vector components of Earth's rotation. In this approach, each gyro sensor was rotated about its rotational axis perpendicular to its axis of sensitivity. The drive unit for rotating the gyro sensors is configured so as to rotate the gyro sensors stably while maintaining a predetermined angular relationship between the input axes of gyro sensors. One drawback to this approach relates to the need for three (3) independent sensor assemblies.
In yet another approach, a gyro-sensor assembly that employs a single gyro was considered. In this approach, the gyro sensor was configured to operate in multiple sequential orientations in order to measure the vector components of Earth's rotation to compute the azimuth of the drilling apparatus. While this approach provides a more compact sensor assembly, other drawbacks become evident. For example, a single gyro sensor configured to be rotated about multiple axes requires a relatively costly and complex multi-axis gimbal apparatus. Moreover, the multiple sequential measurements by a single gyro can take an inordinate amount of time to perform. If such measurements are take every time a drill length is added, significant measurement delays will accrue resulting in a significant increase in the amount of time it takes to drill the borehole (drilling rig costs are usually paid by the hour).
What is needed, therefore, is a method and apparatus for tracking and guiding the drilling of a borehole with increased precision and accuracy. What is also needed is a method and apparatus for tracking and guiding the drilling of a borehole in a generally horizontal path that is disposed under a geographic obstacle (wherein the ground above the borehole is difficult or impossible to access). What is further needed is sensor apparatus configured to determine an azimuthal measurement by obtaining and analyzing a single-vector component of the Earth's rotation vector.